This article introduces the Harmony scientific workbench (SWB), a tool for modeling the multistatic polarimetric ocean observations of the European Space Agency (ESA) Earth Explorer 10 Harmony mission. This work focuses on the retrieval of the mission’s radar-based level-2 (L2) geophysical data products, namely stress-equivalent wind at 10 m height (U10s) and total surface current (TSC) vectors. The retrieval uses simulated level-1 (L1) radar data, which is documented in a companion paper [1]. Different inversions of the simulated L1 observations are tested. We demonstrate that the assumptions made in traditional scatterometric retrieval approaches do not always hold at the high resolutions (O(1 km)) at which Harmony will operate. In particular, wave-Doppler suffers from representation noise, which leads to incorrect estimates of surface currents. The incorporation of the mean cross-section (MACS) and the spectral cutoff parameters improve the estimates of surface current substantially. In addition, iterative estimations with increasing resolution help to reduce the noise in the geophysical retrievals. Although it is possible to get close to 0.3 m/s root mean square error (RMSE) on the wind speed, and 0.1 m/s on the ocean currents at the scale of 1 km×1 km, there are limits to geophysical model function (GMF)-type retrievals, as they are not able to fully account for ocean surface characteristics such as the skewed and wavelength-dependent directional behaviour of wave systems. Despite these limitations, our results indicate that Harmony will provide the world’s first accurate high-resolution ocean wind and current observations based on SAR data.

Abstract This paper summarizes an evaluation by experts of how coordination of Earth‐observing Synthetic Aperture Radar (SAR) missions among the world's space agencies could advance toward game‐changing scientific discoveries and fully realizing SAR's practical capability to address many issues facing society. We consider key science disciplines for which spaceborne SAR sensors are routinely used, with an emphasis on SAR imaging instruments. We outline the current state of the science and identify critical information gaps for 10 disciplines: Ice Sheets and Glaciers, Solid Earth Science, Hazards, Forests and Biomass, Wetlands, Agriculture and Crop Monitoring, Soil Moisture, Sea Ice, Permafrost, and Oceans. We provide recommendations on how these gaps can be addressed by coordination of missions currently operating or in development, then look forward to the next decade during which as‐yet‐unplanned coordinated SAR constellations could be game‐changing. We identify synergies and conflicts between the optimal SAR configurations required for individual disciplines to achieve transformational science advancement. Finally, we provide summary recommendations for beneficial coordination that consider SAR‐enabled Earth science studies both as a whole and within the context of multiple individual disciplines that have benefited from a common observational strategy. Overall, there are clear benefits that can be derived from coordinated utilization of spaceborne SAR assets based on their individual capabilities and availability, and through coordinated and shared data and observation strategies.

This article introduces the Harmony scientific workbench (SWB), a tool for modelling the multistatic, polarimetric ocean observations of the European Space Agency (ESA) Earth Explorer 10 Harmony mission. This work focuses on the simulation of the level-1 (L1) radar observations of the ocean that the mission is expected to produce. These L1 products, along with their modeled errors and uncertainties, are used as input for testing level-2 (L2) retrieval algorithms which invert the radar observables to geophysical variables, such as stress-equivalent wind at 10 m height (U10s) and total surface current (TSC). The L2 retrieval is documented in a companion paper [1]. For Harmony’s L1 normalized radar cross-section (NRCS) and Doppler observations, we extend the Radar Imaging Model (RIM) and Doppler Radar Imaging Model (DopRIM) to account for bistatic geometry and polarimetry. Our implemented forward model relies on wave spectra that are locally altered by wind variations and surface currents. Additionally, cutoff wavelengths and mean cross-section (MACS) are estimated from synthetic aperture radar (SAR) spectra, which are computed using a bistatic mapping function that accounts for polarimetry. The forward models are applied to two ocean scenes to demonstrate the effect of local current and wind-speed variations on the observations. We validate our simulated outputs by comparing against existing C-band geophysical model functions (GMFs). By implementing the RIM, DopRIM and bistatic spectral transfer functions in a consistent manner, this work represents the most complete treatment of the multiscale nature of high-resolution SAR ocean observations to date.

The European Space Agency’s (ESA) BIOMASS mission is a pioneering Earth observation satellite mission launched on April 29, 2025. Utilizing a P-band synthetic aperture radar (SAR), the objective of BIOMASS is to deliver estimates of above-ground forest biomass, forest height (FH), and forest disturbance (FD), with unprecedented accuracy. The mission’s primary scientific goal is to quantify the distribution and changes in forest biomass, thereby reducing uncertainties in carbon flux estimates and informing climate models. The satellite’s advanced instrumentation and innovative approach allow it to penetrate dense forest canopies, capturing data even in challenging environments. The mission will operate in two distinct phases: the tomographic phase and the interferometric phase, which will support polarimetric interferometric SAR (Pol-InSAR) and tomographic SAR (TomoSAR) processing. Additionally, BIOMASS will provide valuable observational data for ice sheets, deserts, the ionosphere, below canopy topography, and other domains.

Highlights What are the main findings? A novel and general method to optimize satellite formations for interferometry applications in a long-baseline scenario like the one foreseen by Harmony mission. Optimal configurations for both low- and high-latitude regions. What are the implications of the main findings? Mission Feasibility. The findings ensure that the Harmony mission can achieve its scientific objectives globally while maintaining safe and fuel-efficient satellite formations. Conceptual Design. The findings contribute to a multistatic SAR mission design, offering insights into the trade-offs between interferometric performance and formation stability.Highlights What are the main findings? A novel and general method to optimize satellite formations for interferometry applications in a long-baseline scenario like the one foreseen by Harmony mission. Optimal configurations for both low- and high-latitude regions. What are the implications of the main findings? Mission Feasibility. The findings ensure that the Harmony mission can achieve its scientific objectives globally while maintaining safe and fuel-efficient satellite formations. Conceptual Design. The findings contribute to a multistatic SAR mission design, offering insights into the trade-offs between interferometric performance and formation stability.Abstract In the framework of Harmony, the 10th ESA Earth Explorer mission, this paper presents a general methodology to optimize the formation parameters relevant to the single-pass, cross-track interferometry (XTI) configuration. The proposed method considers the requested height sensitivity and the maximum allowable temporal lag and derives the formation parameters for an optimal coverage over different ranges of latitudes by leveraging the relative eccentricity and inclination vector formalism. Our approach addresses the problem of interferometric coherence through the wavenumber support alignment method which is able to take into account the specific geometry of XTI in Harmony, which is a long-baseline multistatic configuration with large squint angles. The analysis is completed by an estimate of the propellant budget, required to maintain the optimized formation, which can be used as a further trade-off parameter within the mission design process. The results indicate that the passively stable helix configuration (with relative eccentricity and inclination phase angles set to 90 degrees) provides a robust solution at equatorial and mid-latitude regions with perpendicular baselines up to the order of 1 km and temporal lag below 10 ms. Conversely, for high-latitude and polar regions, two alternative strategies are identified, revealing a trade-off between enhanced interferometric performance and increased formation maintenance requirements. For polar regions, a first strategy adopts relative eccentric and phase angles of 10 degrees, achieving satisfactory performance across most latitudes, whereas an alternative approach retains the value of 90 degrees and optimizes the formation specifically for high latitudes. These two options result in distinct station-keeping demands since the former strategy requires a Delta V budget about two orders of magnitude higher, while the latter remains within a Delta V range that is typical for missions of the considered class.

This paper presents the results of the ESA COBIS4Harmony campaign in which we acquired and analyzed 52 bistatic synthetic aperture radar (SAR) datasets over the Girose glacier using Sentinel-1A as opportunistic transmitter and a stationary ground receiver to better understand the contribution of the Harmony mission to cryosphere research. The acquired datasets are processed to generate bistatic sigma nought images, InSAR coherence and digital elevation models (DEMs). The presented analysis is centered on the evolution of the bistatic sigma nought winter-summer time series and the estimation of the phase center depth using volumetric coherence inversion (Harmony’s baseline algorithm) in comparison with the wet-dry difference InSAR DEM. To facilitate the phase center depth retrieval, a new analytical formulation of the InSAR elevation bias due to snow penetration is introduced for the spaceborne transmitter/stationary receiver geometry, extending the Harmony-bistatic case (with practically the same incidence and backscatter angles) to the COBIS4Harmony configuration, in which the backscatter direction is quite different from the incidence direction.

Selected as European Space Agency’s seventh Earth Explorer in May 2013, the BIOMASS mission will provide crucial information about the state of our forests and how they are changing. This mission is being designed to provide, for the first time from space, P-band Synthetic Aperture Radar measurements to determine the amount of biomass and carbon stored in forests. The data will be used to further our knowledge of the role forests play in the carbon cycle.In this context of an innovative sensor, the concept of Mission Algorithm and Analysis Platform dedicated to the BIOMASS, to the NASA-ISRO SAR (NISAR) mission and to the NASA Global Ecosystem Dynamics Investigation (GEDI) mission mission is proposed. Developed in a collaborative way between ESA and NASA, this Mission Algorithm and Analysis Platform will implement, as part of the payload data ground segment, a virtual open and collaborative environment. The goal is to bring together data centre (Earth Observation and non- Earth Observation data), computing resources and hosted processing, collaborative tools (processing tools, data mining tools, user tools, …), concurrent design and test bench functions, accounting tools to manage resource utilisation, communication tools (social network) and documentation. This platform will give the opportunity, for the first time, to manage the community of users of the BIOMASS mission thanks to this innovative concept.To best ensure that users can collaborate across the platform and to access needed resources, the MAAP requires all data, algorithms, and software to conform to open access and open-source policies. As an example of best collaborative and open-source practices, most of the BIOMASS Processing Suite (BPS) will be made openly available within the MAAP. This Processing Suite contains all elements to generate the BIOMASS upper-level data products and is currently in development under the umbrella of the open-source project called BioPAL. BioPAL is developed in a coherent manner, putting a modular architecture and reproducible software design in place. BioPAL aims to factorize the development and testing of common elements across different BIOMASS processors. The architecture of this scientific software makes lower-level bricks and functionalities available through a well-documented Application Programming Interface (API) to foster the reuse and continuous development of processing algorithms from the BIOMASS user community. This API will greatly simplify the use of the BIOMASS Processing Suite (BPS) on the MAAP.In addition to open satellite data and open-source algorithms, open reference data is needed for Calibration and Validation. GEOTREES is composed of Biomass Reference Measurement sites that are in situ forest measurement sites with a common standard for high-quality data acquisition, transparent measurement protocols, long-term monitoring, and measurements traceable to SI units. GEO-TREES will be established through collaboration with existing international networks of high-quality forest plots that use standard forest monitoring protocols.

The EarthExplorer 10 mission Harmony by the European Space Agency ESA, scheduled for launch around 2029–2030, consists of two passive C-band synthetic-aperture-radar companion satellites flying in a flexible constellation with one Sentinel-1 radar satellite as an illuminator. Sentinel-1 will serve as transmitter and receiver of radar waves, and the two Harmonys will serve as bistatic receivers without the ability to transmit. During the first and last year of the 5-year mission, the two Harmony satellites will fly in a cross-track interferometric constellation, such as that known from TanDEM-X, about 350 km ahead or behind the assigned Sentinel-1. This constellation will provide 12-day repeat DEMs, among other regions, over most land-ice and permafrost areas. These repeat DEMs will be complemented by synchronous lateral terrain displacements from the well-established offset tracking method. In between the cross-track interferometry phases, one of the Harmony satellites will be moved to the opposite side of the Sentinel-1 to form a symmetric bistatic “stereo” constellation with ±~350 km along-track baseline. In this phase, the mission will provide opportunity for radar interferometry along three lines of sight, or up to six when combining ascending and descending acquisitions, enabling the measurement of three-dimensional surface motion, for instance sub- and emergence components of ice flow, or three-dimensional deformation of permafrost surfaces or slow landslides. Such measurements would, for the first time, be available for large areas and are anticipated to provide a number of novel insights into the dynamics and mass balance of a range of mass movement processes.

This article presents a multichannel ground-based bistatic synthetic aperture radar (SAR) receiver architecture designed to perform single-pass tomography using the Sentinel-1 satellites as transmitters of opportunity. The bistatic receiver presents only three imaging channels, which is an extreme case for single-pass tomographic estimation. The three antennas are placed in a nonuniform configuration such that the two antenna separations are in a 2:1 ratio. For a fixed array length, the nonuniform three-element array will extend the maximum unambiguous height (relative to the three-element uniform array) while keeping the elevation resolution cell around the Rayleigh limit. In the proposed processing flow, for each Sentinel-1 overpass on the envisaged orbits, the bistatic SAR image of each channel is focused on a 2-D grid, and afterward, the elevation profile of a given area is computed using the Capon estimator. The proposed architecture was evaluated in a measurement campaign performed between June and November 2021 using an electronic target with two transmit antennas placed on a vertical pole situated 58.5 m from the ground receiver. For an array length of 2.6 m, the overall root mean square error of the relative height was below 10 cm, while the unambiguous interval and the height resolution cell were around 3.75 and 1.2 m, respectively. The experimental data from this measurements campaign provide the first quantitative assessment of spaceborne transmitter/stationary receiver single-pass bistatic SAR tomography in a controlled environment. In the long term, these results may contribute to future multistatic spaceborne SAR missions for which a single-pass tomographic capability is envisioned.

The Harmony satellite mission was recently approved as the next European Space Agency (ESA) Earth Explorer 10. The mission science objectives cover several applications related to solid earth, the cryosphere, upper-ocean dynamics and air–sea interactions. The mission consists of a constellation of two satellites, flying with the Copernicus Sentinel 1 (C or D) spacecraft, each hosting a C-band receive-only radar and a thermal infrared (TIR) payload. From an ocean dynamics/air–sea interaction perspective, the mission will provide the unique opportunity to observe simultaneously the signature of submesoscale upper-ocean processes via synthetic aperture radar and TIR imagery. The TIR imager is based on microbolometer technology and its acquisitions will rely on four channels: three narrow-band channels yielding observations at a ≃1 km spatial sampling distance (SSD) and a panchromatic (PAN, 8–12 μm) channel characterized by a ≃300 m SSD. Our study investigates the potential of Harmony in retrieving spatial features related to sea surface temperature (SST) gradients from the high-resolution PAN channel, relying on top-of-atmosphere (TOA) observations. Compared to a standard SST gradient retrieval, our approach does not require atmospheric correction, thus avoiding uncertainties due to inter-channel co-registration and radiometric consistency, with the possibility of exploiting the higher resolution of the PAN channel. The investigations were carried out simulating the future Harmony TOA radiances (TARs), as well as relying on existing state-of-the-art level 1 satellite products. Our approach enables the correct description of SST features at the sea surface avoiding the generation of spurious features due to atmospheric correction and/or instrumental issues. In addition, analyses based on existing satellite products suggest that the clear-sky TOA observations, in a typical mid-latitude scene, allow the reconstruction of up to 85% of the gradient magnitudes found at the sea-surface level. The methodology is less efficient in tropical areas, suffering from smoothing effects due to the high concentrations of water vapor.

Joint retrieval of vegetation status from synthetic aperture radar (SAR) and optical data holds much promise due to the complimentary of the information in the two wavelength domains. SAR penetrates the canopy and includes information about the water status of the soil and vegetation, whereas optical data contains information about the amount and health of leaves. However, due to inherent complexities of combining these data sources there has been relatively little progress in joint retrieval of information over vegetation canopies. In this study, data from Sentinel-1 and Sentinel-2 were used to invert coupled radiative transfer models to provide synergistic retrievals of leaf area index and soil moisture. Results for leaf area are excellent and enhanced by the use of both data sources (RSME is always less than 0.5 and has a correlation of better than 0.95 when using both together), but results for soil moisture are mixed with joint retrievals generally showing the lowest RMSE but underestimating the variability of the field data. Examples of such synergistic retrieval of plant properties from optical and SAR data using physically based radiative transfer models are uncommon in the literature, but these results highlight the potential for this approach.

Reliable crop monitoring is paramount to achieve the objectives of the Common Agricultural Policy (CAP) and Food and Agriculture Organization. Synthetic Aperture Radar (SAR) provides high-resolution imaging and all-weather data acquisition capabilities for crop monitoring. This study investigates the sensitivity of parcel-level Sentinel-1 interferometric coherence to farming activities (e.g. planting, emergence, harvest and tillage) and weather events. A methodology to detect activities was developed and validated using ground-truth data from four crop types, collected over four years. The proposed approach was able to detect over 60% of all nine different farming activities. The results show that interferometric coherence is a reliable indicator for farming activities that can be considered as events resulting in a clear structural change (e.g. tillage 100%), but less reliable for gradual changes (e.g. Emergence 40%).

Synthetic Aperture Radar (SAR) data handling, processing, and interpretation are barriers preventing a rapid uptake of SAR data by application specialists and non-expert domain users in the field of agricultural monitoring. To improve the accessibility of Sentinel-1 data, we have generated a reduced-volume, multi-year Sentinel-1 SAR database. It includes mean and standard deviation of VV, VH and VH/VV backscatter, pixel counts, geometry, crop type, local incidence angle and azimuth angle at parcel-level. The database uses around 3100 Sentinel-1 images (5 TB) to produce a 12 GB time series database for approximately 770,000 crop parcels over the Netherlands for a period of three years. The database can be queried by Sentinel-1 system parameters (e.g. relative orbit) or user application-specific parameters (e.g. crop type, spatial extent, time period) for parcel level assessment. The database can be used to accelerate the development of new tools, applications and methodologies for agricultural and water related applications, such as parcel-level crop bio-geophysical parameter estimation, inter-annual variability analysis, drought monitoring, grassland monitoring and agricultural management decision-support.

The Biomass mission was selected as the 7th Earth Explorer Mission within the frame of the ESA Earth Observation Programme. The primary objective of Biomass is to determine the worldwide distribution of forest aboveground biomass in order to reduce the major uncertainties in calculations of carbon stocks and fluxes associated with the terrestrial biosphere, including carbon fluxes associated with Land Use Change, forest degradation and forest regrowth. Secondary objectives of the mission include imaging of sub-surface geological structures in arid environments, generation of a true Digital Terrain Model without biases caused by forest cover, measurement of glacier and ice sheet velocities, and better knowledge of the ionosphere. To meet these objectives Biomass will carry, for the first time in space, a fully polarimetric P-band synthetic aperture radar (SAR). This paper provides a description of the mission objectives and system architecture.

This paper describes the rationale and development of the estimation techniques for the level-2 data products of the European Space Agency's 7th Earth Explorer BIOMASS mission. BIOMASS is planned for launch in 2023 and will carry the first-ever P-band synthetic aperture radar (SAR) onboard a satellite. It has been designed to produce consistent global maps of the Earth's forests during a nominal five-year lifetime. Fully polarimetric SAR data will be collected and the satellite orbit will be selected for repeat-pass interferometry and tomography in separated mission phases. Mission requirements call for three level-2 data products: above-ground biomass, forest height and forest disturbance. The paper also discusses the expected limitations of the estimation techniques and remaining problems to be addressed.

Scheduled for launch in 2023, ESA's seventh Earth Explorer Mission, BIOMASS, will carry the first P-band synthetic aperture radar (SAR) to be flown in space, to gather fully polarimetric acquisitions over forested areas worldwide in interferometric and tomographic modes. This paper presents the algorithms developed to estimate biophysical parameters from BIOMASS measurements and their implementation in the BIOMASS level 2 (L2) prototype processor. The L2 processor will generate global maps of forest Above Ground Biomass (AGB), Forest Height (FH), Forest disturbance (FD). Accurate generation of these products requires the L2 processor to be closely inter-linked with the BIOMASS interferometric processor, in order to produce phase-calibrated interferometric stacks, retrieve sub-canopy terrain topography, and generate a 3D representation of forest structure by use of SAR tomography. AGB estimation results are here shown using BIOMASS-like acquisitions derived from campaign data acquired over six tropical forests in South America and Equatorial Africa.

The Biomass mission is the 7th Earth Explorer Mission within the frame of the ESA Earth Observation Programme. The primary objective of Biomass is to determine the worldwide distribution of forest above-ground biomass in order to reduce the major uncertainties in calculations of carbon stocks and fluxes associated with the terrestrial biosphere. To meet these objectives Biomass will carry, for the first time in space, a fully polarimetric P-band synthetic aperture radar (SAR) supporting also interferometric acquisitions. This paper provides an overview of the Biomass system design and development status.

We present a new perspective on Earth's land surface, providing a normalised microwave backscatter map from spaceborne Synthetic Aperture Radar (SAR) observations. The Sentinel-1 Global Backscatter Model (S1GBM) describes Earth for the period 2016-17 by the mean C-band radar cross section in VV- and VH-polarisation at a 10 m sampling. We processed 0.5 million Sentinel-1 scenes totalling 1.1 PB and performed semi-automatic quality curation and backscatter harmonisation related to orbit geometry effects. The overall mosaic quality excels (the few) existing datasets, with minimised imprinting from orbit discontinuities and successful angle normalisation in large parts of the world. Regions covered by only one or two Sentinel-1 orbits remain challenging, owing to insufficient angular variation and not yet perfect sub-swath thermal noise correction. Supporting the design and verification of upcoming radar sensors, the obtained S1GBM data potentially also serve land cover classification and determination of vegetation and soil states. Here, we demonstrate, as an example of its potential use, the mapping of permanent water bodies and evaluate against the Global Surface Water benchmark.

Using a combination of multi-directional SAR and TIR measurements, the Harmony Earth Explorer 10 mission candidate will provide high resolution simultaneous measurements of surface stress, surface currents SST and wave spectra over oceans, 3-D deformation vectors over solid Earth, and time-series of surface elevation changes over volcanic areas and land ice masses. This will serve a series of science objectives aimed at better understating multi-scale processed and feedbacks in the Earth System.